The present invention relates to improving the adhesion characteristics of a copper surface to enable subsequently applied coatings to be completely and easily adhered to the copper surface and, more particularly, to an improved cleaning and adhesion promoter composition and method for treating the surface of copper foil laminates used in printed circuits to improve the adhesion of subsequently applied coatings affixed to the copper surface.
The composition of the invention removes contaminants from the copper surface (which in turn is adhered to an underlying substrate) and enhances the surface topography thereof by controlled conversion of the copper surface from a relatively smooth surface to a micro-roughened surface, through a microetching which removes, e.g., from about 0.5 to 5 microns of the copper. In this way, a copper foil laminate for use as a printed circuit board is provided with improved adhesion characteristics to enable subsequently applied photoresist or oxide coatings to more easily and completely adhere to the copper surface. More particularly, the invention relates to an improved cleaning and adhesion promoter composition which is capable of concurrently bringing about both cleaning and microetching of the copper surface to improve the adhesion characteristics of the copper surface prior to the application of photoresist or oxide coatings to the copper surface in the manufacture of printed circuit boards.
In a typical preparation of printed circuits, a copper foil is bonded to an insulating substrate which may be phenolic, epoxy, polyimide, polyester, or the like. The foil is normally treated electrolytically to provide a specific surface structure, such as disclosed in U.S. Pat. Nos. 3,293,109; 3,318,758; 3,518,168; 4,049,481; 4,131,517; 4,176,035; and others.
Multilayer printed circuit boards are assemblies containing several copper foil-clad substrate inner-layers, further bonded to each other through layers of semicured polymeric material, which are subsequently cured at elevated temperatures and pressures to form the complete laminate assembly. Prior to this assembly, the copper foil of the innerlayers is imaged and etched to form the innerlayer circuits of the assembly.
The printed circuit board industry is continuously faced with an increasing demand for higher density circuitry. This has led to the production of finer lines of circuitry and is leading toward the use of thinner and thinner copper foils. As this trend continues, the preparation of the innerlayer copper foil becomes increasingly more critical. The use of thinner foils requires much tighter processing controls in order to produce uniform circuitry.
Combining the steps of initially cleaning and preparing a copper surface adhered to an underlying substrate to improve the adhesion characteristics of the copper surface for easily and completely affixing subsequently applied coatings would reduce the labor involved as well as the time to process the circuit boards. Thus, the final cost of manufacturing each board would be reduced.
While the present invention is particularly apropos of printed circuit board manufacture, and more particularly innerlayers of multilayered circuit boards, the need to remove contaminants from copper surfaces and to improve the adhesion characteristics of copper surfaces so that subsequently applied coatings will be more completely adhered to the copper surface also arises in other contexts where a contaminant-free surface with adhesion-improved characteristics of the copper surface is needed for subsequent plating operations for decorative or functional purposes.